1. Field of the Invention
The present invention relates to a liquid crystal display device that includes an alignment film for aligning liquid crystal molecules in a liquid crystal layer which is sandwiched between two substrates.
2. Description of the Related Art
Liquid crystal display devices are being applied to increasingly wide range of uses because of their features such as high display quality, thinness, light weight, and low power consumption. The various uses of liquid crystal display devices include monitors for portable devices, e.g., cell phone monitors and digital still camera monitors, monitors for desktop personal computers, monitors for printing and designing, monitors for medical practice, and liquid crystal television sets. The expansion of uses has brought about demands for liquid crystal display devices that are even higher in image quality and product quality, and there are particularly strong demands for higher luminance, which is to be accomplished by enhancing the transmittance, and for lower power consumption. A demand for lower cost is also increasing as liquid crystal display devices grow popular.
A liquid crystal display device usually displays an image or the like by applying an electric field to liquid crystal molecules in a liquid crystal layer sandwiched between a pair of substrates, which changes the alignment direction of the liquid crystal molecules and consequently changes the optical characteristics of the liquid crystal layer. The direction in which the liquid crystal molecules are aligned during no-electric field application is controlled by an alignment film obtained by subjecting a surface of a thin polyimide film to rubbing treatment. Conventionally, an active drive liquid crystal display device, which includes a switching element such as a thin-film transistor (TFT) for each pixel, includes electrodes on both of two substrates paired to sandwich a liquid crystal layer, in order to apply a vertical field, i.e., an electric field oriented substantially perpendicular to the substrate plane, to the liquid crystal layer, and displays an image or the like by utilizing the optical rotatory power of liquid crystal molecules that constitute the liquid crystal layer. A typical vertical field type liquid crystal display device is a twisted nematic (TN) liquid crystal display device. One of major problems of TN liquid crystal display devices is their narrow viewing angle. Known display methods that widen the viewing angle include in-plane switching (IPS) and fringe field switching (FFS). IPS and FFS are lateral field display methods in which a comb tooth-shaped electrode is formed on one of paired substrates and a generated electric field includes a component substantially parallel to the substrate plane. An IPS or FFS liquid crystal display device displays an image or the like by rotating liquid crystal molecules that constitute a liquid crystal layer within a plane substantially parallel to substrates and utilizing the birefringence of the liquid crystal layer. IPS and FFS liquid crystal display devices, which involve in-plane switching of liquid crystal molecules, have such advantages over conventional TN liquid crystal display devices as wider viewing angle and smaller load capacity and, in recent years, are advancing rapidly as promising new liquid crystal display devices to replace TN liquid crystal display devices.
Whichever display method a liquid crystal display device uses, an image sticking phenomenon of a displayed image happens in some cases. Japanese Patent Application Laid-open No. 2008-216858 states that FFS liquid crystal display devices are prone to the image sticking phenomenon due to the asymmetric electrode structure, and describes a solution to this matter.